As an apparatus for discharging or dispensing a liquid in a constant amount, there is known an apparatus that sucks the liquid into a measuring bore (cylinder) with driving of a piston, and that discharges the liquid having been sucked into the measuring bore with the driving of the piston. Regarding that type of apparatus, there have been proposed so far various apparatuses each including a plurality of pistons and a plurality of measuring bores. The apparatus including the plural pistons and the plural measuring bores is designed such that a time wastefully taken to suck and discharge the liquid in successive steps is reduced by alternately sucking and discharging the liquid. As examples of the apparatuses each including the plural pistons and the plural measuring bores, there are apparatuses in each of which one multi-way value is used to switch over channels connected to the plural measuring bores, a liquid supply port, and a liquid discharge port. Those apparatuses are mainly grouped, depending on the piston type, into an apparatus in which a liquid is sucked and discharged at one side of the piston as disclosed in Patent Document 1, and an apparatus in which a liquid is sucked and discharged at both sides of the piston as disclosed in Patent Document 2.
A method disclosed in Patent Document 1 is as follows. A paint cylinder is disposed in a paint pressure-feed line through which a highly-viscous paint is fed under pressure, and a hydraulic cylinder is disposed in such a state that respective piston rod heads of the paint cylinder and the hydraulic cylinder are coupled to each other. The highly-viscous paint fed under pressure from a paint pressure-feed pump is supplied to the paint cylinder and is temporarily stored therein. Hydraulic operating oil is then injected into the hydraulic cylinder to extend a piston rod of the hydraulic cylinder, thereby contracting a piston rod of the paint cylinder. As a result, the highly-viscous paint stored in the paint cylinder is supplied to an airless painting apparatus through the paint pressure-feed line.
An apparatus disclosed in Patent Document 2 is as follows. A pair of plunger pumps having the same diameter are coaxially arrayed such that respective open ends of their cylindrical bores are opposed to each other. Plungers are integrally coupled to each other in a state where a spacing distance between both cylinders is adjustable. By alternately repeating an operation of supplying a pressurized liquid to an outlet tube of one pump and an operation of opening an outlet tube of the other pump to ambient air, the integrally-coupled plungers are caused to reciprocally slide in both the cylindrical bores due to a difference between pressures of liquids being present in both the cylindrical bores and acting on the plungers. As a result, a constant amount of the liquid is alternately discharged through the outlet tube that is opened to the ambient air. By adjusting sliding strokes of the plungers depending on the spacing distance between both the cylinders, the amount of the liquid alternately discharged from the output tubes of both the pumps can be set to a desired value.
Meanwhile, the applicant has proposed, in Patent Document 3, a bubble mixing prevention mechanism for use in a discharge apparatus comprising a measuring portion that has a channel communicating with a nozzle, and a plunger that is reciprocally moved in the channel of the measuring portion, the bubble mixing prevention mechanism being mountable to an end of the measuring portion at the side opposite to the nozzle, wherein the bubble mixing prevention mechanism includes a first bore that is communicated with the channel of the measuring portion and that allows the plunger to reciprocally move therein, a first sealing member that is disposed at an end of the first bore at the side closer to the nozzle, a second sealing member that is disposed at an end of the first bore at the side opposite to the nozzle, and a second bore that is communicated with a lateral surface of the first bore.